In a medical field, a radiation tomographic apparatus (ECT: Emission Computed Tomography) is used, in which radiation emitted from a radioactive drug administered in a subject and localized in a region of interest is detected, and the tomographic image of the radioactive drug distribution in the region of interest in the subject is obtained. As such popular ECTs, a PET (Position Emission Tomography) apparatus and a SPECT (Single Photon Emission Computed Tomography) apparatus are known.
As an example, a PET apparatus will be described below. A PET apparatus is an apparatus configured to form a PET image showing a distribution of radioactive drug labeled with positron-emitting nuclides in the subject. As shown in FIG. 16, the PET apparatus 51 is provided with a plurality of radiation detectors 53 arranged so as to surround the subject M in a ring-shape. The radioactive drug administered to the subject is accumulated in a region of interest, and positrons are emitted from the accumulated drug. The emitted positron causes an annihilation with an electron, resulting in release of two γrays (gamma rays), i.e., a γ-ray N1 (gamma ray N1) and a γ-ray N2 (gamma ray N2), for one positron. Since the γ-ray N1 (gamma ray N1) and the γ-ray N2 (gamma ray N2) have momentum opposite with each other, they are emitted in opposite directions and simultaneously detected by the respective radiation detectors 53.
Based on the positional information of the detected γ-rays, the position where the annihilation occurred, i.e., the position of the radioactive drug, is calculated and accumulated as positional information. Then, based on the accumulated positional information, an image showing the distribution of the radioactive drug in the region of interest is provided by the PET apparatus (See, e.g., Patent Document 1 and Non-patent Document 2).
In recent years, besides the PET apparatus, a TOF-PET (TOF: Time of Flight) apparatus has been used to perform diagnosis. In the TOF-PET apparatus, by measuring the difference of flight times of two γ-rays (gamma rays) from the radiation position to the detection position by both the radiation detectors, the generation position of the γ-rays (gamma rays) is identified. Since the TOF-PET apparatus uses the information on the time difference, the apparatus can obtain a distribution image of the radioactive drug having less noise than a normal PET apparatus.
In some cases, as a radiation detector used in such PET apparatus, a radiation detector having a structure capable of performing a depth position discrimination of the scintillator provided therein is mounted (see Patent Documents 2 and 3). FIG. 17 is a perspective view showing a structure of a conventional radiation detector. In this conventional radiation detector 100, a scintillator block 101, a light guide 103, and a solid state light detector 105 are laminated in this order.
The scintillator block 101 is constituted by scintillator crystal layers 101a, 101b, 101c, and 101d each formed by two-dimensionally integrating rectangular parallelepiped scintillator crystals. The scintillator crystal layers 101a, 101b, 101c, and 101d are laminated in the z-direction, i.e., in the depth direction of the scintillator block 101, and configured to emit light by absorbing γ-ray (gamma rays) emitted from a subject. The light emitted in the scintillator block 101 will be referred to as “scintillator light”.
The light guide 103 is optically coupled to the scintillator block 101 and the solid state light detector 105 and transmits the scintillator light to the solid state light detector 105. The solid state light detector 105 is made of, e.g., two-dimensionally arranged SiPM (Silicon Photo Multiplier) elements as examples of light receiving elements, and is configured to detect the scintillator light transmitted by the light guide 103 and convert the scintillator light into electrical signals. Based on the converted electrical signals, a tomographic image showing the distribution of the positron-emitting nuclides in a region of interest is acquired. With this structure, the light source position in the depth direction at which interaction occurred (DOI: Depth of Interaction) can be discriminated.